Part 15: What should we expect? Future climate projections

August 14, 2012

By Luisa Cristini, PhD, University of Hawaii at Manoa

Note from the editor: This is the fifteenth and last in a series of blog entries that focused on introductory topics in climate dynamics and modeling, and served to provide insight into the current understanding of the science.]

The last house on Holland Island, Maryland, where 360 people lived before tides took over. Photograph by Astrid Riecken for the Washington Post/Getty Images retrieved from National Geographic Daily News.

Given the increasing evidence of climate change, what should we expect our future climate to be like? A large number of simulations are available from a broad range of climate models. Together with additional information from observations, they provide a quantitative basis for estimating likelihoods for many aspects of future climate change. Model simulations run for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) in 2007 cover a range of possible futures based on possible greenhouse gases emissions. The IPCC assessment relies on a large number of climate models of increasing complexity and realism, as well as information on the feedbacks from the carbon cycle and climate observations.

Surface air temperature: Numerical model experiments show that even if carbon dioxide emissions were held constant at year 2000 levels, a global surface air temperatureincrease would occur in the next two decades at a rate of about 0.1°C per decade, due mainly to the slow response of the oceans. About twice as much warming is expected when emissions are within the range of the SRES scenarios (discussed in Part 14). Best-estimate projections from models indicate that decadal average warming over each continent by 2030 is independent of the scenario chosen and is very likely (probability of occurrence higher than 90%) to be at least twice as large as the corresponding model-estimated natural variability during the 20th century. Continued greenhouse gas emissions at or above current rates will cause further warming and induce changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century. In addition, increase in temperature tends to reduce land and ocean uptake of atmospheric carbon dioxide, further increasing the fraction of anthropogenic emissions that remains in the atmosphere. The projected warming in the 21st century shows scenario-independent geographical patterns as observed over the past decades. Warming is expected to be greatest over land and at most high northern latitudes, and least over the Southern Ocean and parts of the North Atlantic Ocean.

Precipitation: Increases in the amount of precipitation are very likely in high latitudes (toward Earth’s poles) while decreases are likely in most subtropical land regions (closer to the Equator).

Extremes: It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent.

Tropical cyclones: It is likely (probability of occurrence higher than 66%) that future tropicalcyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and heavier precipitation associated with ongoing increases of tropical sea surface temperatures. There is less confidence in projections of a global decrease in numbers of tropical cyclones. Extra-tropical storm tracks are projected to move poleward, with consequent changes in wind, precipitation and temperature patterns.

Sea level rise: Model-based estimates of global sea level rise at the end of the 21st century (excluding future rapid dynamical changes in ice flow from the Greenland and Antarctic ice sheets), range between 0.18 and 0.59 m, relatively to the average sea level between 1980-1999. The ocean’s thermal expansion is projected to be a major contributor to this rise.

Ocean acidification: Increasing atmospheric carbon dioxide concentrations lead to increasing ocean acidification, as explained in the article “Ocean acidification and the ’short-term‘ marine carbon cycle” by German marine geologist and PhD candidate Franziska Kersten. Projections based on SRES scenarios give reductions in average global surface ocean pH (corresponding to increased acidity) of between 0.14 and 0.35 units over the 21st century, adding to the present decrease of 0.1 units since pre-industrial times.

Sea ice: Sea ice (the ice formed on the ocean surface when the water freezes) is projected to shrink in both the Arctic and Antarctic. In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century.

Meridional Overturning Circulation (MOC): It is very likely that the MOC, a system of surface and deep ocean currents, of the Atlantic Ocean will slow down. The multi-model average reduction by 2100 is 25%. This could mean a decrease in warm water reaching the eastern coasts of the North Atlantic Ocean (e.g., Europe). However, temperatures in the Atlantic region are projected to increase despite such changes due to the much larger warming associated with projected increases in greenhouse gases. It is very unlikely (probability of occurrence less than 10%) that the MOC will undergo a large abrupt transition during the 21st century.

Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. Based on current understanding, model studies suggest that to stabilize carbon dioxide concentration at 450 ppm (parts per million) could require that cumulative emissions over the 21st century be reduced from an average of approximately 670 GtC (gigatons of carbon, i.e., billions tons of carbon) to approximately 490 GtC.

Greenland Ice Sheet: Contraction of the Greenland Ice Sheet is projected to continue to contribute to sea level rise after 2100. Ice-sheet models suggest that mass losses increase with temperature more rapidly than gains due to precipitation and the surface mass balance becomes negative (meaning that the ice sheet loses mass) at a global average warming in excess of 1.9°C to 4.6°C relative to pre-industrial values. A negative surface mass balance sustained for millennia would lead to virtually complete elimination of the Greenland Ice Sheet and a resulting contribution to sea level rise of about 7 m.

Antarctic Ice Sheet: Current model studies project that the Antarctic Ice Sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if ice discharge dominates the ice sheet mass balance.

Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the time scales required for removal of this gas from the atmosphere and the slow response of major climate components such as oceans and ice sheets.